The researchers collected samples from a randomized controlled trial evaluating the efficacy of IIV-HD (60mcg HA per strain) vs. IIV-SD (15mcg HA per strain) in adults aged ≥65 years available for testing, in which one- third of trial participants provided post-vaccination sera. This substudy utilized a case-cohort design, in which 675 representative samples collected during the 2012–2013 season of the study (from individuals who either developed polymerase chain reaction/culture confirmed H3N2 influenza illness [N=123] or belonged to a random subset of 10% of non-cases [N=552]) were selected for expanded testing.

Expanded immunogenicity was assessed with an HAI assay using an MDCK cell-propagated A/Victoria/361/2011 (H3N2) antigen, a viral neutralization assay (NT) using both egg- and cell-propagated A/Victoria/361/2011 (H3N2) antigens, and an enzyme-linked lectin assay (ELLA) for anti-neuraminidase (N2) antibodies. Geometric mean titers (GMT) were estimated for IIV-HD and IIV-SD recipients using weighted averages and were compared as GMT ratios (IIV-HD/IIV-SD).

This study, which utilized two additional assays beyond the previously studied HAI assay, confirmed earlier findings that (IIV-HD) confers superior immunogenicity in older adults, as compared to IIV-SD. The link between the “explored antibody titers and vaccine protection against influenza is yet to be established,” Dr. DiazGranados noted.